Conventionally, a permanent magnet rotational electric motor is known in which there are provided a first rotor and a second rotor that are provided concentrically around an axis of rotation of an electric motor, and the relative positions in the circumferential direction of the first rotor and the second rotor, namely, the phase difference thereof are controlled in accordance with the rotational speed of the rotational electric motor, or in accordance with the speed of a rotating magnetic field that is generated in a stator (see, for example, Japanese Unexamined Patent Application, First Publication No. 2002-204541).
In addition, conventionally, a motor is known in which there are provided a first permanent magnetic pole piece and a second permanent magnetic pole piece which can alter mutual phase positions thereof by a servo pressure, and an amount of field flux thereof can be altered (see, for example, Japanese Unexamined Patent Application, First Publication No. S55-153300).
In the motor according to the above conventional art, there is a concern that motor power cannot be appropriately controlled if a sensor which detects a phase position does not work, in a case in which the motor power is controlled in accordance with a detection value of the sensor.
For example, if using the nonworking sensor is stopped and controlling of phase is forbidden, the phase position cannot be known. As a result, the available power of the motor according to the phase position cannot be known, and the available power which can be controlled is excessively decreased. Therefore, desired amount of power cannot be maintained.